1. Field of the Invention
This invention relates to novel blocked polyisocyanates which are soluble or dispersible in water and contain cyanurea anions in a chemically bound form as groups which ensure their solubility or dispersibility, to a process for the preparation of these blocked polyisocyanates and to their use in the form of a solution or dispersion in water as hardeners in two-component lacquers.
2. Description of the Prior Art
Blocked polyisocyanates which are soluble or dispersible in water are known, for example, from DE-A-2,456,469. These compounds may be used as aqueous solutions or dispersions in combination with water soluble or dispersible polyhydroxyl compounds to serve as binder components for polyurethane lacquers which are applied from the aqueous phase.
In two-component polyurethane lacquers, the "lacquer polyisocyanates+ which are used as binder components constitute the comparatively low molecular weight activator for the relatively high molecular weight polyhydroxyl compounds. In the interests of obtaining satisfactory cross-linking and high quality coatings, these activators should have an average functionality of at least 2.5, preferably at least 3.
In the process for the preparation of the known water soluble or dispersible blocked polyisocyanates, however, a significant proportion of the isocyanate groups present in the polyisocyanates used as starting material is used up by the hydrophilic modification (reaction of a proportion of the isocyanate groups with an isocyanate-reactive hydrophilic component). The cross-linking capacity of the polyisocyanates is thereby considerably reduced.
It is therefore an object of the present invention to provide water soluble or dispersible polyisocyanates in which the hydrophilic group which renders the polyisocyanates soluble or dispersible takes part in the cross-linking reaction so that it becomes incorporated in the polyisocyanates and thereby preserves their cross-linking potential.
This problem is solved by providing the blocked polyisocyanates according to the invention described below and a process for their preparation. The hydrophilic centers present in the blocked polyisocyanates according to the invention serve a double function. They ensure that the polyisocyanates are soluble or dispersible in water and also take part in the crosslinking reaction at the same time losing their hydrophilic character.
Polyisocyanates containing anionic cyanurea groups have been described in EP-A-0,185,184, but these polyisocyanates are not self-cross-linking compounds containing blocked isocyanate groups. There is no indication in this publication that the Principle of cross-linking disclosed therein could be combined with the known type of chemical cross-linking of polyhydroxyl compounds with blocked polyisocyanates as is the case when using the blocked polyisocyanates according to the invention.